FIG. 1 illustrates how V.34 modems combine a pre-coding scheme with a four-dimensional Trellis coding scheme. As shown in FIG. 1, only the signal y(n), not u(n), must have Trellis code properties (i.e., y(n) should be Trellis code sequence) since it is the signal received by the receiver and decoded by the Trellis decoder. V.34 modems make y(n) to be a Trellis code sequence by making sure that u(n) is a Trellis sequence and u(n) (precoder input) and y(n) (precoder output) is in the same four-dimensional (4D) family as defined in L. F. Wei, "Trellis-Coded modulation with Multidimensional Constellations," IEEE Trans. on Information Theory, vol. 33, no. 4, July 1987. As long as u(n) and y(n) are in the same four-dimensional family, making u(n) a Trellis sequence guarantees that y(n) is also a trellis sequence.
Current state-of-the-art V.90 modems use a faster (56 kbps) pulse code modulation (PCM) coding scheme for downstream transmission and the slower (33.6 kbps) V.34 coding scheme for upstream transmissions. Upstream PCM transmission would employ a different pre-coding scheme, and therefore the technique used for V.34 can no longer be used.
Thus, a need exists for a method and apparatus for combining a Trellis coding scheme with a pulse code modulation transmission for lower error probability at a given upstream transmission rate.